﻿[ 
  694 
  J 
  

  

  L 
  VI. 
  On 
  the 
  Effect 
  of 
  Pressure 
  on 
  the 
  Electric 
  Wind 
  from 
  

   a 
  Discharging 
  Point. 
  By 
  J. 
  R. 
  Boon, 
  B.Sc* 
  

  

  THE 
  pressure 
  of 
  the 
  electric 
  wind 
  from 
  a 
  discharging 
  point 
  

   in 
  various 
  gases 
  has 
  been 
  measured 
  by 
  Chattock 
  f 
  , 
  

   Chattock 
  and 
  Tyndall 
  J, 
  and 
  Tyndall 
  §. 
  

  

  Thus 
  it 
  has 
  been 
  shown 
  by 
  Chattock 
  for 
  various 
  gases, 
  and 
  

   later 
  by 
  Chattock 
  and 
  Tyndall 
  for 
  mixtures 
  of 
  hydrogen 
  and 
  

   oxygen, 
  that 
  if 
  a 
  point 
  is 
  placed 
  opposite 
  a 
  plane 
  at 
  a 
  distance 
  

   z 
  from 
  it, 
  the 
  average 
  pressure 
  on 
  the 
  plane, 
  p, 
  for 
  a 
  given 
  

   current 
  density 
  is 
  a 
  linear 
  function 
  of 
  z 
  when 
  the 
  values 
  of 
  

   the 
  latter 
  are 
  of 
  the 
  order 
  of 
  a 
  few 
  centimetres. 
  This 
  is 
  in 
  

   accordance 
  with 
  the 
  theoretical 
  deductions 
  based 
  on 
  the 
  

   assumptions 
  that 
  the 
  ions 
  move 
  viscously 
  and 
  that 
  the 
  dis- 
  

   charge 
  is 
  a 
  one-way 
  discharge. 
  If, 
  however, 
  the 
  experimental 
  

   values 
  of 
  jo 
  and 
  z 
  are 
  plotted 
  and 
  the 
  line 
  is 
  produced 
  tojo 
  = 
  0, 
  

   it 
  cuts 
  the 
  axis 
  of 
  z 
  at 
  some 
  positive 
  value, 
  referred 
  to 
  in 
  what 
  

   follows 
  as 
  z 
  . 
  

  

  For 
  negative 
  discharge 
  in 
  u 
  pure' 
  ; 
  hydrogen 
  z 
  was 
  as 
  

   much 
  as 
  4 
  centimetres, 
  but 
  it 
  rapidly 
  decreased 
  to 
  3 
  or 
  4 
  

   millimetres 
  on 
  the 
  addition 
  of 
  small 
  percentages 
  of 
  oxygen 
  

   or 
  air. 
  For 
  positive 
  discharge 
  under 
  the 
  same 
  conditions 
  it 
  

   was 
  3 
  or 
  4 
  millimetres 
  throughout. 
  In 
  air 
  z 
  Q 
  was 
  found 
  to 
  

   have 
  about 
  the 
  same 
  value, 
  4 
  millimetres, 
  for 
  both 
  signs 
  of 
  

   discharge. 
  Later, 
  Tyndall 
  showed 
  that 
  the 
  distance 
  z 
  was 
  

   independent 
  of 
  the 
  current 
  and 
  of 
  the 
  size 
  of 
  the 
  point. 
  

  

  The 
  simplest 
  explanation 
  of 
  the 
  existence 
  of 
  z 
  is 
  that 
  

   through 
  a 
  certain 
  portion 
  of 
  the 
  distance 
  between 
  point 
  and 
  

   plane 
  the 
  discharge 
  is 
  either 
  wholly 
  or 
  partially 
  non- 
  wind- 
  

   producing 
  ; 
  the 
  later 
  experiments 
  seemed 
  to 
  locate 
  this 
  region 
  

   near 
  the 
  point. 
  In 
  order 
  to 
  obtain 
  further 
  experimental 
  data 
  

   the 
  author 
  has 
  made 
  measurements 
  of 
  the 
  value 
  of 
  z 
  at 
  

   different 
  pressures 
  and 
  in 
  different 
  gases. 
  From 
  the 
  results 
  

   some 
  definite 
  conclusions 
  may 
  be 
  drawn, 
  and 
  these 
  are 
  set 
  

   down 
  below. 
  

  

  Apparatus. 
  

  

  The 
  main 
  experimental 
  difficulty 
  is 
  the 
  construction 
  of 
  a 
  

   gauge 
  of 
  high 
  sensitiveness 
  to 
  measure 
  the 
  small 
  differences 
  of 
  

   pressure 
  involved. 
  The 
  Chattock-Fry 
  gauge, 
  which 
  had 
  been 
  

  

  * 
  Communicated 
  "by 
  Dr. 
  A. 
  M. 
  Tyndall. 
  

  

  t 
  Chattock, 
  Phil. 
  Mag. 
  [5] 
  xlviii. 
  p. 
  401. 
  

  

  X 
  Chattock 
  & 
  Tyndall, 
  Phil. 
  Mag. 
  [6] 
  xix. 
  p. 
  449. 
  

  

  § 
  Tyndall, 
  Phil. 
  Mag. 
  [6] 
  xxi. 
  p. 
  585. 
  

  

  